This invention relates FeRAM and DRAM integrated circuits, and specifically to structures that have Irxe2x80x94Taxe2x80x94O, Irxe2x80x94Tixe2x80x94O, Irxe2x80x94Nbxe2x80x94O, Irxe2x80x94Alxe2x80x94O, Irxe2x80x94Hfxe2x80x94O, Irxe2x80x94Vxe2x80x94O or Irxe2x80x94Zrxe2x80x94O as bottom electrodes and PGO thin film on top of these electrodes for applications.
PGO thin film refers to Pb5Ge3O11, ferroelectric phase. Although c-axis PGO usually exhibits layered microstructure, during the deposition process it is difficult to form single phase c-axis PGO thin film having a very smooth and uniform surface. One reason is that the PGO phase is polycrystalline. However, there are a few other lead germanium oxide compounds, which are very close to the Pb5Ge3O11 phase, both in composition and formation temperature, and which are easier to form under similar conditions. If multiple phase lead germanate, having different microstructures, is formed on the surface of a bottom electrode at the same time, it is difficult to obtain a smooth and uniform c-axis PGO thin film. Several factors affect the formation of single-phase, c-axis PGO thin film, one of which is the surface condition of the bottom electrodes. The lattice constant matching is an important factor to form layered c-axis PGO thin film. The microstructure of PGO phase is hexagonal structure having lattice constants of a=10.251 xc3x85 and c=10.685 xc3x85. For pure iridium (Ir) and platinum (Pt) metal bottom electrodes, which are face-centered-cubic (FCC) structures having lattice constants of a=3.83 xc3x85 and axe2x88x923.92 xc3x85, respectively. Theoretically, it is relatively difficult to obtain the c-axis PGO single-phase on both electrodes. However, while this is true for a Pt substrate, c-axis PGO film may be formed relatively easily on an Ir substrate. This may be due to the thin layer of IrO2 which forms on the Ir surface in situ during the deposition and annealing process, which may assist the c-axis PGO nucleation and grain growth. IrO2 has lattice constants of a=4.498 xc3x85, c=3.154 xc3x85.
The orientation of the bottom electrode is also very important for the phase formation of the PGO thin film. It has been found that amorphous and polycrystalline substrates promote the formation of a smooth and uniform PGO thin film. A strong oriented substrate, having mismatched lattice constants tends to promote formation of polycrystalline ferroelectric PGO thin film having other secondary phases, wherein the film exhibits a rough surface.
FIG. 1 depicts a PGO thin film formed on a patterned substrate by MOCVD. The light area in FIG. 1a is a polished Pt substrate area, the darker areas are SiO2 substrate. Both types of substrates are polished and planarized. FIG. 1bdepicts the crystalline structure of a PGO thin film formed on the Pt (left) and SiO2 (right) substrate. It is clearly seen that the PGO thin film formed on the Pt substrate is polycrystalline and exhibits a rough surface. The PGO thin film formed on the SiO2 substrate exhibits a layered single-phase structure. The PGO thin film formed on the SiO2 substrate is single-phase c-axis PGO thin film.
The thermal stability of the electrode is also important in order to form a smooth and uniform single-phase c-axis PGO thin film. It has been found that both Pt and Ir tend to form hillocks during high temperature annealing, e.g., above 500xc2x0 C., which affects the nucleation and orientation of PGO thin film. An Ir composite electrode, however, is very stable during even very high temperature annealing in oxygen ambient.
The existence of oxygen in the bottom oxide electrode also plays an important role. Because both the PGO and bottom electrode are metal oxide, the favored bonding condition between the oxides at the interface can increase nucleation density help in the formation of a smooth c-axis PGO thin film.
Fengyan Zhang, Tingkai Li, Douglas J. Tweet and Sheng Teng Hsu, Phase and microstructure analysis of lead germanate thin film deposited by metalorganic chemical vapor deposition, Jpn. J. Appl. Phys. Vol. 38, pp59-61 1999, discusses various phases of lead germanate as formed in thin films.
Fengyan Zhang, Jer-shen Maa, Sheng Teng Hsu, Shigeo Ohnish and Wendong Zhen, Studies of Irxe2x80x94Taxe2x80x94O as high temperature stable electrode Material and its application for ferroelectric SrBi2Ta2O9 thin film deposition, Jpn. J. Appl. Phys. Vol. 38, pp1447-1449, 1999, describes the use of a Ta barrier layer and an Irxe2x80x94Taxe2x80x94O electrode.
Fengyan Zhang, Tingkai Li, Tue Nguyen, Sheng Teng Hsu, MOCVD process of ferroelectric lead germanate thin films and bottom electrode effects, Mat. Res. Soc. Symp. Proc. Vol. 541, pp549-554, 1998, describes growth of c-axis PGO thin film.
A method of forming an electrode and a ferroelectric thin film thereon, includes preparing a substrate; depositing an electrode on the substrate, wherein the electrode is formed of a material taken from the group of materials consisting of iridium and iridium composites; and forming a single-phase, c-axis PGO ferroelectric thin film thereon, wherein the ferroelectric thin film exhibits surface smoothness and uniform thickness. An integrated circuit includes a substrate; an electrode deposited on the substrate, wherein the electrode is formed of a material taken from the group of materials consisting of iridium and iridium composites, wherein the iridium composites are taken from the group of composites consisting of IrO2, Irxe2x80x94Taxe2x80x94O, Irxe2x80x94Tixe2x80x94O, Irxe2x80x94Nbxe2x80x94O, Irxe2x80x94Alxe2x80x94O, Irxe2x80x94Hfxe2x80x94O, Irxe2x80x94Vxe2x80x94O, Irxe2x80x94Zrxe2x80x94O and Irxe2x80x94O; and a single-phase, c-axis PGO ferroelectric thin film formed on the electrode, wherein the ferroelectric thin film exhibits surface smoothness and uniform thickness.
An object of this invention is to provide a uniform, single-phase, c-axis PGO thin film on a metal electrode.
Another object of the invention is to provide an iridium composite electrode, such as IrO2, Irxe2x80x94Taxe2x80x94O, Irxe2x80x94Tixe2x80x94O, Irxe2x80x94Nbxe2x80x94O, Irxe2x80x94Alxe2x80x94O, Irxe2x80x94Hfxe2x80x94O, Irxe2x80x94Vxe2x80x94O, Irxe2x80x94Zrxe2x80x94O or Irxe2x80x94O, as bottom electrode for FeRAM and DRAM applications.
Still another object of the invention is to provide a method of forming a PGO thin film on a metal electrode which may be used in integrated circuits, such as capacitors, pyroelectric infrared sensors, optical displays, optical switches, piezoelectric transducers, and surface acoustic wave devices.
A further object of the invention is to provide a method for depositing a PGO thin film by chemical solution deposition (CSD), sputtering, MOCVD or other thin film deposition methods, which will exhibit the smoothness and uniformity desired in the fabrication of an integrated circuit.
Yet another object of the invention is to provide an iridium composite electrode to improve the surface characteristics and lattice structure of a PGO thin film.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.